The present invention relates to a low NOx combustor for a gas turbine based power plant. More specifically, the present invention relates to a combustor for burning a hot coal-derived fuel gas in an gas turbine based power plant.
The high efficiency, low capital cost and short lead time of gas turbine based power plants make them particularly attractive to electric utilities as a means of producing electrical power. Unfortunately, traditionally, gas turbines have been limited to operation on expensive, sometimes scarce, fuels--chiefly, distillate oil and natural gas. As a result of the ready availability and low cost of coal, considerable effort has been expended toward developing a gas turbine system for generating electrical power that can utilize coal as its primary fuel.
Several approaches have been developed toward accomplishing this end. One approach utilizes what is often referred to as "first generation" pressurized fluidized bed combustion. According to this approach, compressed air from a gas turbine compressor is used to fluidize coal in a pressurized fluidized bed combustor ("PFBC") and the resulting hot gas is expanded in a turbine to produce power. Unfortunately, the thermodynamic efficiency of this approach is poor due to the need to limit the bed temperature and, hence, the temperature of the air entering the turbine, to approximately 870.degree. C. (1600.degree. F.) in order to optimize the capture of the sulfur in the coal and avoid the carryover of harmful alkali vapors into the turbine. This is in contrast to modern conventional gas or liquid fuel fired gas turbines, which can operate with turbine inlet gas temperatures in excess of 1425.degree. C. (2600.degree. F.). As is well known in the art, increasing the temperature of the gas entering the turbine section increases the power output and efficiency of the gas turbine.
Therefore, in a second a approach, often referred to as a "second generation" pressurized fluidized bed combustion, a portion of the compressed air from the compressor is used to generate a low heating value fuel gas in a gasifier. This low heating value fuel gas is then combusted in a topping combustor, thereby achieving the optimum hot gas temperature for expansion in the turbine. A second portion of the air from the compressor is used to fluidize coal, or char from the gasifier, in the PFBC. The vitiated hot air discharged from a PFBC is then directed to the topping combustor where it serves as combustion air for the low heating value fuel gas.
In still another approach, often referred to as an "integrated gasified combined cycle" ("IGCC") power plant, a portion of the compressed air is used to generate a fuel gas in a gasifier, which is then burned in a topping combustor, as discussed above. However, a PFBC is not utilized and the compressed air not utilized in the gasifier is sent directly to the topping combustor.
Although the use of a topping combustor in the second generation fluidized bed combustion and IGCC processes improves thermodynamic performance, the combustion of the fuel gas can result in the formation of nitrogen oxides ("NOx"), which are considered atmospheric pollutants.
Accordingly, it would be desirable to provide a combustor capable of combusting a fuel while generating a minimum of NOx.